The present invention relates to improvement in electronically controlled fuel injection systems for internal combustion engines.
A known system of this type is constructed as shown in FIG. 1 so that an air flow sensor 2 for measuring the amount of air flow to an engine generates an electric signal V.sub.S corresponding to the displaced position of a baffle plate 4 disposed in an intake pipe or manifold 3 of the engine, and the electric signal is applied to a control circuit 6 which in turn controls the duration of opening of fuel injection valves 7 or the amount of fuel injected into the engine.
In the FIG. 1, when a throttle valve 5 is closed rapidly or in a manner close to the rapid closing upon deceleration of the engine, due to the inertia of the air flowing through the baffle plate 4 of the air flow sensor 2 just before the rapid closing, a certain amount of air momentarily flows from the downstream side of the baffle plate 4 into a portion A upstream of the throttle valve 5 so that the pressure in the portion A is increased and a force having a direction indicated by an arrow B in the Figure is applied to the baffle plate 4. Thus, the baffle plate 4 is displaced in a direction tending to decrease the amount of air flow, generating a signal V.sub.S which is not corresponding to the actual amount of air flow to the engine (namely, a signal indicative of an air quantity smaller than the actual air flow rate) and then its output returns to the normal condition. Thus, in such a case, the ratio of air quantity to fuel quantity or the air-fuel ratio (hereinafter referred to as an A/F ratio) first deviates to the large (or lean) side of the desired value so that the engine misfires or the engine nearly misfires and the output torque of the engine decreases, and then at the expiration of a certain time the output signal returns to the normal condition so that the torque is increased, thus moving the vehicle jerkily and causing a feeling of unpleasantness on the part of the driver. Another disadvantage is that the occurrence of engine misfiring results in the emission of harmful exhaust gases. FIG. 2 (a), (b) and (c) respectively show variations in the amount of the throttle valve opening, in the amount of air flow to the engine and variations in the output signal of the air flow sensor 2 in relation to time axis. When the throttle valve 5 is closed rapidly at a time X as shown in (a) of FIG. 2, the amount of air flow to the engine does not change rapidly as shown in (b) of FIG. 2 due to a large volume of the portion between the throttle valve 5 and the engine intake valves. However, while there will be no problem if the output signal V.sub.S of the air flow sensor 2 varies as shown by the broken line in (c) of FIG. 2, the signal in fact varies as shown by the solid line in (c) of FIG. 2 for the previously mentioned reasons and the A/F ratio deviates to the large side by an amount corresponding to the hatched portion. The air flow sensor 2 is so designed that a voltage V.sub.S is generated which is proportional to the opening angle of the baffle plate 4, that is, the opening angle is increased with an increase in the amount of air flow to the engine and the resulting output voltage ia also increased with respect to the ground potential, whereas the output voltage is decreased with a decrease in the amount of air flow to the engine.
In addition, the time corresponding to the hatched portion of the output signal of the air flow sensor 2 shown in (c) of FIG. 2 varies depending on the operating conditions of the engine. FIG. 3 shows variations in the A/F ratio upon deceleration according to different rates of deceleration. The starting point of deceleration is indicated at X as in the case of FIG. 2. When the engine is decelerated from a constant speed driving operation, the A/F ratio slightly deviates to the large side as shown by the curve A, whereas when the engine is decelerated rapidly after a rapid acceleration or under a condition which may be considered to be the severest one the A/F ratio deviates to the large side very greatly as shown by the curve C and also the drivability is deteriorated. Each of the A/F curves attains the highest magnitude immediately after the deceleration and it comes nearer to the correct normal value with the lapse of time.